Chapter 12A: BASIC THERMODYNAMICS AND LAWS Agami Reddy (rev-May 2017) 1) Basic thermodynamic concepts 2) Changes of state 3) Sensible and latent heat 4) First law of thermo for closed systems- sensible/ latent heat 5) First law for open systems- concept of enthalpy 6) Second law of thermodynamics and entropy (much of this lecture material is a review of Chapter 2A slides) HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Basic Thermodynamics It is important to know the fundamentals of thermodynamics since it is basic to understanding how thermal fluid devices and systems operate. Wide range of applications: Cooling and heating systems use water and air Coal plants and nuclear use water/steam as working fluid Natural gas use turbines use compressed air Gasoline and diesel engines use air HCB 3- Chap 12A: Basic Thermo and Laws

Property, Process and Cycle System: objects that you are studying (defined by you!). Property: characteristic of system such as temperature, pressure,… State: condition of a system as described by its properties. Any property change RESULTS in state change Process: a change in state (one or more properties change). It is related to path followed Extensive and intensive properties: A property is intensive if it does not depend on the amount of the matter present (such as pressure, specific volume, specific enthalpy, temperature,….). Volume is an example of an extensive property. Property is not process related! Cycle: one or more processes wherein the initial and final states of the system are identical; i.e. all the properties have the same value. HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Ideal and real gases Pure substances and mixtures Determining properties steam tables Process: the change in two properties usually as a result of work done or heat added: Work= P .dV Process path represents the change of pressure and volume during compression- it is shown on on a p-v diagram Fig. 12.1 p-V diagram of an Expansion process HCB 3- Chap 12A: Basic Thermo and Laws

Changes of State (Phase) Substances can exist in three states: solid, liquid, and gas (vapor) Two factors that affect state: temperature and pressure Process of state change - Temperature change or phase change - Pressure dependent Molecular Theory of Liquids and Gases suggested to explain observed phenomena Concepts of “saturated state” - subcooled - superheated We use saturated steam tables to determine properties- discussed later HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Change of State Molecular theory or Kinetic theory Temperature is a measure of average molecule speed Some molecules have faster speed and escape Resisting pressure plays a role During boiling process, average speed reaches a level at which the link between molecules break HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Internal Energy HCB 3- Chap 12A: Basic Thermo and Laws

First Law of Thermodynamics A closed system is one where the fluid does not cross the system boundaries Energy Balance or energy conservation law Different ways to express it Closed System: Change in the internal energy U is the difference in the heat Q added to the system minus work done by the system If no work involved: the change in total energy in a system equals the energy added to the system minus the energy removed from the system dU = Qin – Qout dU: change in internal or stored energy in the system Qin: heat added (entering ) to the system Qout: heat removed (leaving) from the system Sign: +ve if energy added to system –ve if energy removed from system dU Qin Qout HCB 3- Chap 12A: Basic Thermo and Laws

Sensible Heat and Latent Heat Why the need to distinguish between the two types of heat processes? During a process with phase change, temperature is not the only variable that determines heat transfer rate. What is sensible heat? Energy (heat) that is added or removed during a process where the temperature of a substance changes but there is no change in state (phase) of the substance. What is latent heat change? Energy (heat) absorbed or released during a process involving state (phase) change. HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Sensible Heat Sensible heat of a body is the energy associated with its temperature. Example- heating water For liquids and solids, the change in stored energy when its mass undergoes a temperature change is given by: where Qs is the stored energy, (kJ or Btu) is the mass (kg or lbm) c is the specific heat, (kJ/kg-oC or Btu/lbm-°F) The same equation also applies when a fluid flow is involved. In that case, Q is the rate of heat transfer (kW or Btu/h) and m is the mass flow rate (kg/s or lbm/h) HCB 3- Chap 12A: Basic Thermo and Laws

Specific Heat and Heat Capacity HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Example The temperature of 150 lb of water in a hot water tank is to be raised by 100 oF. The heat content of one ft3 of natural gas is 1,000 Btu/ ft3. If the efficiency of the water heater is 80%, how many ft3 of natural gas needs to be burnt? Amount Of heat HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Latent Heat How to calculate latent heat at a given temperature Inter-molecular force changes – Enthalpy changes although temperature does not change Use latent heat equation: where Q is the heat transfer rate, W or Btu/h m is the mass flow rate, kg/s or lbm/h hfg is the latent heat of vaporization, kW or Btu/lbm This is given as the difference between: hf is the enthalpy of saturated liquid, kJ/kg or Btu/lbm hg is the enthalpy of saturated vapor, kJ/kg or Btu/lbm HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Enthalpy (h) A property of a body that measures its heat content Enthalpy includes: (i) Internal energy U and (ii) pV or energy due to flow work Enthalpy is a combined property which is widely used in thermal analysis When T, p or V changes, H changes Enthalpy H = U + V.p in kJ or Btu (V is total volume) specific enthalpy h = u + p.v in kJ or Btu/lbm (v is specific vol.) Instead of sensible or latent heat equations, enthalpy equation is widely used since one does not have to worry about state of fluid: T P V P- pressure V- volume T- temperature HCB 3- Chap 12A: Basic Thermo and Laws

First Law of Thermodynamics contd. Fig. 2.8 For a OPEN system: Extension of Bernouilli equation (2.30) HCB 3- Chap 12A: Basic Thermo and Laws

First Law of Thermodynamics contd. For a OPEN system where KE, PE and other energy sources are negligible - when W=0: Q = m (hout – hin) boiler - when Q=0: W = m (hin– hout) turbine where hin: enthalpy of fluid entering the system hout: enthalpy of fluid leaving the system Sign convention for Q: +ve when added to the system –ve when removed from the system Sign convention for W is opposite HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Example Example 2.3: Cooling of air supplied to room Consider a room which contains 300 kg (660 lbm) of air. If this air is to be refreshed once each hour, and cooled from 29.4o C (85o F) to 12. 8o C (55o F), what is the cooling capacity of the cooling coil? Specific heat of air cp =1.00 kJ/(kg.oC) Assumptions: Steady flow process, neglect changes in kinetic and potential energies, no work is involved, density of air remains constant, duct is adiabatic   Solution Since there is no change of phase, we can use the sensible heat eqn: HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Applications of First Law of Thermodynamics Efficiencies of components of a system can be multiplied to yield total System efficiency From Randolph and Masters, 2008 HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws From Randolph and Masters, 2008 HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Source: EIA, Annual Energy Review 2004, DOE/EIA-0384(2004) (Washington, D.C., August 2005), Diagram 5. HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws All forms of energy are not equal! Thermal energy (or heat) is a more “disordered” form of energy- From Cengal and Boles HCB 3- Chap 12A: Basic Thermo and Laws

Laws of Thermodynamics In simplest terms, the Laws of Thermodynamics dictate the specifics for the movement of heat and work. Basically, the First Law of Thermodynamics is a statement of the conservation of energy – the Second Law is a statement about the quality of energy or direction of that conservation – and the Third Law is a statement about reaching Absolute Zero (0 K). However, since their conception, these laws have become some of the most important laws of all science - and are often associated with concepts far beyond what is directly stated in the wording. Heat is the lowest form of energy Work (from which electricity is produced) is a higher form One unit of thermal energy at a high temperature is more VALUABLE than the same amount of energy at a lower temperature HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Entropy General observational statements: a) Any system which is free of external influences becomes more disordered with time. b) The irrevocable loss of some energy to the environment can be associated with an increase of disorder in that system. The 19th century physicist, Clausius, proposed the use of a variable to quantify disorder- entropy Clausius worked out a general equation devoted to the measurement of entropy change over a period of time: entropy S = dQ / T (the change in entropy is equal to the amount of heat added to the system [which is an irreversible process] divided by the temperature in Kelvin). HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Entropy contd… Concept of entropy: a measure of the irreversibility of the process (due to friction, heat transfer across a temperature difference,…) a property (just like temperature, pressure, enthalpy) Entropy acts as a function of the state of a system : - high amount of entropy translates into higher chaos within the system, while - low entropy is reflective of a highly ordered state. do not use fuel sources which can give you high temperatures for low temperature applications- for example, using solar energy for hot water heating is a better sustainability practice than using gas or electricity Proper understanding important for energy resources sustainability HCB 3- Chap 12A: Basic Thermo and Laws

HCB 3- Chap 12A: Basic Thermo and Laws Outcomes Understanding of basic thermodynamic definitions such as system, process, cycle,… Understanding of different states and the kinetic theory of gases Understanding of basic thermodynamic properties such as internal energy, enthalpy and entropy Understanding the difference between sensible and latent heat and be able to solve simple problems Understanding of how the first law is a representation of the conservation of energy concept Be able to apply the first law of thermodynamics to closed and open system analysis Familiarity with the applications and insights provided by the second law Familiarity with the concept of entropy HCB 3- Chap 12A: Basic Thermo and Laws